Laser system with reduced apparent speckle

ABSTRACT

Laser systems with reduced apparent speckle are provided. The laser systems emit laser light having different mode structures that change within a time period of an integration period of an imaging system used to observe a field of view that is at least in part illuminated by the laser systems.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/929,762 filed Jan. 21, 2014.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO A “SEQUENCE LISTING”

Not applicable.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to laser systems, and more particularly tolaser illuminator systems.

Description of Related Art

Laser illumination systems project a beam of collimated light across anarea. Often this is done to enable observation of a laser illuminatedarea in a particularly useful but limited range of wavelengths. This canbe done for example to illuminate an area with light that is not visibleto people but that can be detected electronically such as byilluminating an area with near infrared light or short wave infraredlight, or this can be done to sense objects in a scene that may befluoresce when illuminated when exposed to specific wavelengths oflight.

One problem with observing laser illuminated areas is while a laserillumination may be generally uniform, non-specular surfaces in theilluminated area may reflect the coherent light from the laser such thatinterference patterns arise when the light is observed by a person orelectronic imager. The interference creates areas that appear to beunnaturally bright and areas that appear to be unnaturally dark creatinga high noise component in the reflected light observed in an area. Theinterference effect is known as speckle.

Speckle is visually distracting and can make it difficult for both humanobservers and automatic vision systems to detect contrast patterns inthe illuminated areas.

Saloma, et al. in a paper entitled “Speckle reduction by wavelength andspace diversity using a semiconductor laser”, published in AppliedOptics, Vol. 29, No. 6, (Optical Society of America 1990) describe aspeckle reduction system that uses modulation of a laser to createadditional longitudinal modes, with each mode having a different laserfrequency. In operation, mode hopping is used and a grating is used tointroduce a shift in a position of a point of a source of theillumination as a function of the change in frequency during the modehopping. The change in position reduces the extent of the specklecontrast when averaged over time.

Trisnadi, in a paper entitled “Speckle contrast reduction in laserprojection displays”, published in Projection Displays VIII, Ming H. Wu,Editor Proceedings of SPIE Vol. 4657, (SPIE 2002) describes generallyspeckle reduction strategies as methods for averaging N independentspeckle configurations with the spatial and temporal resolution of adetector and identifies three different mechanism for speckle reduction:wavelength diversity which requires a laser with a sufficiently largerange of wavelengths to reduce speckle, polarization diversity whichrequires emission of laser light having two different polarizations andangle diversity which requires shifting the point of illumination.Trisnadi proposes a combination of polarization and angle diversity toachieve speckle reduction. In Trisnadi, angle diversity is accomplishedusing a moving diffuser.

Geske et al. U.S. Pat. No. 8,743,923 describe the use of amulti-wavelength VCSEL array to reduce speckle using wavelengthdiversity. In this embodiment, the VCSEL array has a plurality of laseremitters each with a different wavelength creating a laser emitterhaving a broad enough bandwidth to reduce the speckle effects.

What is need in the speckle reduction art is a solid state laser devicethat does not require the grating and extended optical path of Saloma,that does not require moving parts like the moving diffuser of Trisnadiand that does not require the complexity and cost of a VSCEL array.

SUMMARY OF THE INVENTION

Laser systems and methods are provided. In one aspect a laser system hassemiconductor laser that is adapted to emit a beam coherent light whensupplied with an electrical current and driving circuit adapted tosupply a first current to the semiconductor laser and to modulate thecurrent supplied to semiconductor across a range of current levelswithin a determined integration time. The current is modulated so thatsemiconductor laser will emit light having a first transverse modestructure during a first portion of the range of current levels and asecond transverse mode structure during a second portion of the range ofcurrent levels causing a shift in the position of a speckle patternduring the integration time that reduces the appearance of speckle.

In another aspect, a method for operating a laser system is provided inwhich an integration time is determined for an imaging system to be usedwith the laser system and a current is supplied to a semiconductor laserused in the laser system. The current supplied to the semiconductorlaser is modulated across a range of current levels during thedetermined integration time and the current is modulated so that thesemiconductor laser will emit light having a first transverse modestructure during a first portion of the range of current levels and asecond transverse mode structure during a second portion of the range ofcurrent levels causing a shift in the position of a speckle patternduring the integration time that reduces the appearance of speckle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a first embodiment of a laser systemaccording to a first embodiment.

FIG. 2 is a top view of one embodiment of the laser system of FIG. 1.

FIG. 3 is an end view of the embodiment of FIG. 2.

FIG. 4. is a side section of the embodiment of FIGS. 1-3.

FIG. 5 shows a first embodiment of a laser system, an imaging system anda field of view.

FIG. 6 shows a speckle pattern.

FIG. 7 shows a flow chart of one method for operating a laser system.

FIG. 8 shows a portion of the speckle pattern on a plurality ofradiation sensors.

FIG. 9 shows the portion of FIG. 8 in combination with a shifted portionof the speckle pattern.

FIG. 10 shows the net effect of the shift on the plurality of radiationsensors.

FIG. 11 shows the effect of unshifted speckle on the plurality ofradiation sensors as is known in the prior art.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic view of first embodiment of a laser system 100.FIGS. 2 and 3 illustrate respectively top and end views of theembodiment FIG. 1. In the embodiment illustrated in FIGS. 1-3 lasersystem 100 has a system housing 102 that encompasses, substantiallyencloses, or otherwise retains, a laser module 104, a drive circuit 106,a system controller 108, a user input system 110, sensors 112, a useroutput system 114, a communication system 116, and a power supply 118.

In this embodiment, system controller 108 receives signals from userinput system 110, sensors 112, and communication system 116 anddetermines whether a response to such signals is required. When systemcontroller 108 determines to respond to received signals by causing alaser emission, system controller 108 sends signals to drive circuit 106causing drive circuit 106 to supply electrical energy from power supply118 to laser module 104 in a manner that causes laser module 104 to emita laser beam 122. System controller 108 can also generate signals thatcause user output system 114 to generate a human perceptible output.Additionally, system controller 108 can send signals to communicationsystem 116 causing communication system 116 to send signals to otherdevices, to cause communication system 116 to receive signals from otherdevices or both. Power supply 118 provides electrical energy to drivecircuit 106, system controller 108, user input system 110, sensors 112,user output system 114, and communication system 116. As is shown inFIGS. 1-3, in this embodiment system housing 102 provides an enclosurefor each of the components of laser system 100 to provide an enclosed astand-alone device capable of laser emission.

System housing 102 can be formed of any of a variety of rigid materialssuch as composites, laminates, plastics or metals. In one configuration,system housing 102 can be formed of an extruded aluminum, therebyproviding sufficient strength without requiring significant weight whilealso providing good thermal transfer properties.

System housing 102 can be fabricated or assembled in any of a variety ofways. In one embodiment, system housing 102 is machined such as by EDM(electrical discharge machining), assembled, or molded if composites,laminates, plastics or metals are employed for system housing 102.System housing 102 also can be fabricated using other conventionaltechniques including but not limited to additive assembly techniques.

In FIGS. 1-3, system housing 102 is shown having a generally cylindricalprofile. However, in other exemplary embodiments, system housing 102 maybe configured to provide surfaces that enable system housing 102 to bejoined, fixed, held, mounted or otherwise positioned for movement withother devices such as hand-held weapon system 14 or to any of a varietyof direct fire weapons such as handheld, side, and small firearms. Suchfirearms include, but are not limited to, pistols, rifles, shotguns,automatic arms, semi-automatic arms, rocket launchers and select grenadelaunchers bows. In other embodiments, system housing 102 can beconfigured to mount any known dismounted or dismounted crew-servedweapon, such as machine guns, artillery, recoilless rifles and othertypes of crew served weapons.

In still other embodiments, system housing 102 can be shaped, sized orotherwise provided in forms that more readily interface with any of avariety of clamping or mounting mechanisms such as a Weaver-stylePicatinny rail or dove tail engagement for mounting to these firearms.In further exemplary embodiments, system housing 102 can be configuredas a component part of a hand-held weapon system 12 or other direct fireweapon, such as a foregrip, sight or stock.

Drive circuit 106 receives power from power supply 118 and controlinputs from system controller 108. In response to the control inputsreceived from system controller 108, drive circuit 106 generates signalsthat cause laser module 104 to emit laser light. In the embodiment thatis illustrated in FIG. 1 laser module 104 is not directly connected topower supply 118 but rather receives power by way of drive circuit 106such that drive circuit 106 can control the time, duration, andintensity of electrical energy supplied to laser module 104. Drivecircuit 106 may be configured to assist in tuning and/or otherwisecontrolling the output of laser module 104. Drive circuit 106 can beconstructed to provide either pulsed or continuous operation of lasermodule 104. The rise/fall time of the pulse, compliance voltage andcurrent generated by drive circuit 106 for the laser module 104 areselected based at least in part upon power consumption, heat generationand desired beam intensity considerations. These parameters may also beselected to cause laser module 104 to produce a beam having a desirablewavelength, frequency, transverse mode number and/or other quantifiablecharacteristics.

Depending on the desired output, drive circuit 106 can enable operationof the laser module 104 as a continuous or pulsed laser, such as bypassive, active, or controlled switching. Although specific valuesdepend upon the particular laser module 104 and intended operatingparameters, it is contemplated the peak power draw of drive circuit 106may be between approximately 1 amp and approximately 10 amps, with anaverage current draw between approximately 0.1 amps and approximately1.0 amps. As the required voltage may be between on averageapproximately 9 volts and approximately 12 volts, approximately 0.9 W toapproximately 12 W may be consumed. This may represent a substantialpower consumption as well as heat generation.

In an exemplary embodiment, drive circuit 106 may assist in controllingand/or modifying the power level of laser module 104 to aid inpenetrating components or conditions of the atmosphere through whichlaser system 100 will direct laser beam 122. Such components orconditions may include, for example, snow, rain, fog, smoke, mist,clouds, wind, dust, gas, sand, and/or other known atmospheric orairborne components. For example, drive circuit 106 can be configured tocontrollably, manually, and/or automatically increase the current and/orvoltage directed to strengthen and/or intensify laser beam 122 emittedby laser module 104 in such conditions.

It is also understood that laser module 104 can have more than onesemiconductor laser 180. In one exemplary embodiment of this type, alaser module 104 can have one semiconductor laser 180 in the form of amid-range adapted infrared quantum cascade laser and anothersemiconductor laser 180 in the form of a long-range adapted infraredquantum cascade laser. Other combinations of semiconductor lasers 180are possible.

Alternatively, in other embodiments, laser module 104 can includecomponents that can receive signals from drive circuit 106 and that canadjust power supplied to laser module 104 in response to such signals.In such an alternative embodiment, laser module 104 may receive mayreceive electrical energy directly from power supply 118.

In the embodiment illustrated in FIGS. 1-3 system housing 102 hasplurality of openings shown as openings 120, 124, 126 and 128. Incertain embodiments, seals 140, 142, 144, 146 can be supplied to providea barrier to resist entry of contaminants at openings 120, 124, 126 and128 so as to protect the components disposed within system housing 102from water, dust, vapors, or other harmful contaminants commonlyexperienced in non-controlled environment use. Optionally, systemhousing 102 can be hermetically sealed, at least in part around lasermodule 104.

User input system 110 includes human operable sensors such as switches,touch pads, joysticks, audio, video, keypads, key locks, proximitysensors or any other known types of sensors that can detect a user inputaction and that can provide signals to system controller 108 indicativeof the user input action. In the embodiment of FIGS. 1-3, user inputsystem 110 provides a switch 130 that takes the form of a four positionmode switch with different settings to enable manual selection of threedifferent operating mode selections and an off selection.

Sensors 112 can include any form of device that can be used to detect orotherwise sense conditions inside or outside of system housing 102 thatmay be useful to system controller 108 in determining actions to betaken in operating laser system 100. Sensors 112 can include withoutlimitation, light sensors such as photovoltaic cells, contact switches,opto-electronic sensors such as light beam emitter and sensor pairs,electro-mechanical sensors such as limit switches, strain sensors, andproximity sensors such as Hall effect sensors, thermal sensors,meteorological sensors, such as humidity sensors, accelerometers,orientation sensors and other known sensors and transducers.

User output system 114 can include, without limitation actuators, lightemitters, video displays, or other sources of human perceptible visual,audio or tactile signals from which a user can determine for example,and without limitation, a status of laser system 100, an operating modeof laser system 100, or that laser system 100 is emitting a laser beam122 and a characteristics of the laser beam 122 that laser system 100 isemitting or will emit when instructed to do so. In this embodiment, useroutput system 114 includes a video display 132 that is positioned inopening 128.

Communication system 116 can include any combination of knowncommunication circuits including wired or wireless transponders,transceivers, transmitters, receivers, antennas, modulators,de-modulators, encryption and de-encryption circuits or software and canprovide other known components to facilitate data communication, theexchange of control signals or power exchanges in wired or wirelessform.

Power supply 118 is shown located within system housing 102. In oneconfiguration, power supply 118 comprises a battery and system housing102 can include a battery compartment (not shown) sized to operablyreceive and retain a power supply 118 in the form of batteries.Depending upon the anticipated power requirements, available space, andweight restrictions, the batteries can be N-type batteries or AA or AAAbatteries. Additionally, a lithium/manganese dioxide battery such asmilitary battery BA-5390/U, manufactured by Ultralife Batteries Inc. ofNewark, N.Y. can be used with laser system 100. The battery-type powersupply 118 can be disposable or rechargeable. Battery compartment can beformed of a weather resistant, resilient material such as plastic, andshaped to include receptacles for receiving one or more batteries orother power storage devices. Further, the battery compartment may beselectively closeable or sealable to prevent environmental migrationinto the compartment or to create a hermetically sealed environmenttherein.

In other exemplary embodiments, power supply 118 can take the form of afuel cell, capacitive system or other portable electrical energy storageor generation system. It is understood that any type of power supply118, preferably portable and sufficiently small in size can be utilized.

As is noted above, system controller 108 drives operation of lasersystem 100 and receives signals from user input system 110, sensors 112and communication system 116 that system controller 108 can use tocontrol operation of laser system 100. System controller 108 comprisefor example a computer, a microprocessor, micro-controller, programmableanalog logic device or a combination of programmable or hardwiredelectronic devices capable of performing the functions and actionsdescribed or claimed herein.

In the embodiment of FIGS. 1-3 system controller 108 determines a modeof operation of laser system 100 in response to a position of switch130. When switch 130 is positioned in the “Off” position, user inputsystem 110 sends signals to system controller 108 causing systemcontroller 108 to remain in an inactive state or can maintain a lowpower consumption mode of operation.

However, when system controller 108 receives signals from user inputsystem 110 indicating that switch 130 has been moved to the “On”position system controller 108 can generate signals causing drivecircuit 106 to drive laser module 104 to generate laser light. In otherembodiments, switch 130 can comprise a switch that provides power toinitiate operation of system controller 108 only when switch 130 is in aposition other than the “Off” position.

Other modes of operation are possible. For example a “Stand By” mode ofoperation can be provided to conserve stored energy of from power supply118 while maintaining the laser system 100 in an advanced state ofreadiness for use. For example, when switch 130 is moved to the “StandBy” position user input system 110 can send signals to system controller108 from which system controller 108 can determine that this mode ofoperation has been selected.

In one embodiment, system controller 108 can detect that switch 130 hasbeen moved to the “Stand By” position and can respond to this by sendingsignals to drive circuit 106 causing drive circuit 106 to beginsupplying power circuits or subsystems, if any, that require some timeto reach a state where they are ready for immediate activation whenswitch 130 is moved to the “On” position. Not all circuits or subsystemswill need be activated at such times and a stand by option relieves theoperator from being confronted with the choice of operating the lasersystem 100 in a high power consumption “On” mode prior to the need to doso and the choice of holding the device in the “Off” state to conservepower with the understanding that there will be a lag time beforeactivation.

Additionally, in the embodiment of FIGS. 1-3 switch 130 can bepositioned at a location that indicates that laser system 100 is to beoperated in a “Test” mode. In one example of this type system controller108 can cause laser module to emit a lower powered laser beam 122. Thislower powered laser beam can 122 be used to allow verification of theoperational status of laser system 100 such as by emitting a lowerpowered laser test beam that can be directed at, for example, nearbytargets for training purposes or at target strips or pages that changein appearance when illuminated by the laser in the test mode. Here too,this mode will be entered when system controller 108 receives a signalfrom user input system 110 indicating that switch 130 has been moved toa position selecting the “Test” mode.

Turning now to FIG. 4 what is shown is a cross-section schematic view ofone embodiment of a laser module 104 taken as shown in FIG. 1. In theembodiment that is illustrated in FIG. 4, laser module 104 has a lasercore 150 with a base 152 having a front side 154 from which a header 156extends in a first direction 160 and a housing 170 shaped to combinewith front side 154 to form a sealed environment about header 156.

A semiconductor laser 180 is mounted to header 156. In this embodiment,semiconductor laser 180 is mounted to header 156 by way of a submount182 and is positioned to direct a divergent laser light 184 in firstdirection 160 through a window 172 on a front portion 174 of housing170. Semiconductor laser 180 or submount 182 can be joined to header 156in any of a variety of ways including conventional fasteners, solders,conductive adhesives and the like. Semiconductor laser 180 in turn istypically bound to submount 182 using soldering techniques, althoughother techniques are also known.

Semiconductor laser 180 can comprise for example, any semiconductordevice that can emit a laser output. Examples of semiconductor laser 180include but are not limited to a diode laser, quantum cascade lasers,inter-band cascade lasers. These types of semiconductor lasers 180 sharegenerally the characteristics of being made from a semiconductormaterial and having a emitting a divergent laser light beam while alsogenerating a meaningful amount of heat that must be dissipated toprotect semiconductor laser 180.

In the embodiment illustrated in FIG. 4, semiconductor laser 180 emits adivergent laser light 184 having a wavelength in the infrared regionsuch as between 2μ and 30μ wavelength. However, in other embodiments,semiconductor laser 180 can emit a divergent laser light 184 having anyof a wide range of wavelengths including but not limited to ultravioletwavelengths, visible wavelengths, and near infrared wavelengths. For thepurposes of the following discussion, it will be assumed that in theembodiment of FIG. 4, semiconductor laser 180 is a quantum cascade typelaser.

A frame 200 is joined to base 152 and extends from base 152 past window172 to position a lens 210 at a distance along axis 162 fromsemiconductor laser 180. In operation, semiconductor laser 180 generatesa divergent laser light 184 which is directed toward lens 210.

Lens 210 collimates the divergent laser light 184 from semiconductorlaser 180 into a laser beam 122 when positioned at a location where lens210 can effectively focus light from semiconductor laser 180. As usedherein a laser beam 122 includes a laser beam that is fully collimatedas well as laser beams having substantial collimation with a limitedallowable divergence.

In general, lens 210 controls the field of illumination provided bydivergent laser light 184. This field of illumination can be narrow soas to concentrate divergent laser light 184 to create a field ofillumination at a distant target or it can be made even more narrow toprovide pointing, marking or designation spots of high intensity. Lens210 can comprise one or more lenses and lens systems and can beadjustable between multiple configurations to provide different degreesof collimation.

Lens 210 is most effective when held within a preferred range ofpositions from semiconductor laser 180. However, in practical use thisis difficult to achieve with a static lens mounting design. Inparticular it will be understood that a variety of forces can conspireto influence the distance that a mechanical system such as frame 200will position lens 210. Chief among these are the forces of thermalexpansion and contraction which can cause significant changes in thelength of components of frame 200 and the resultant position of lens 210relative to semiconductor laser 180.

In this embodiment, frame 200 is optionally of an athermalized designmeaning that frame 200 is designed so that frame 200 will hold lens 210in a desirable range of positions relative to semiconductor laser 180despite any thermal expansion or contraction of any components of frame200 that may arise during transport and operation of laser system 100.Such systems do not seek to completely resist or prevent heating orcooling of frame 200, but rather are defined to provide mechanisms toallow for automatic compensation for any thermal expansion caused bysuch heating or cooling.

Optionally, frame 200 can be configured to allow a user to adjust thedegree of collimation of divergent laser light 184 so as to form a beamoutput 122 having a divergence that is within a range of divergences.This adjustability can allow laser system 100 to be used for a varietyof functions including but not limited to illuminating a relativelynearby field of view and a relatively distant field of view, and provinga highly collimated beam for designating, marking or pointing purposes.

As is shown in FIG. 5, laser beam 122 from laser system 100 illuminatesa field of illumination 237 and light 240 can reflect therefrom in aspecular fashion or by way of scattered reflection, be absorbed therebyor be absorbed and re-emitted thereby 242. Additionally, field of view238 may have portions thereof that emit or reflect light 244 other thanthat provided by laser beam 122 and that are at or near wavelengths tothose emitted by the illuminator or that can otherwise be sensed by athermal imaging system such as thermal imaging system 248.

FIG. 5 one exemplary embodiment of a thermal imaging system 248 havingan optional lens system 250 and a thermal imager 252. Lens system 250focuses light from a field of view 238 to form an image onto thermalimager 252. Thermal imager 252 is configured to sense a range ofwavelengths of light including wavelengths of light emitted by lasersystem 100. As is illustrated here, field of illumination 237 and fieldof view 238 at least in part overlap.

Thermal imager 252 may be any device or combination of devicesconfigured to receive such reflected light 240, re-emitted light 242 andother light 244. Conventionally, thermal imager 252 has an imagingsurface 254 with an array of radiation sensors 256. In a typicalconfiguration, individual radiation sensors 256 are each capable ofgenerating a signal that is representative of an amount of radiationincident on the radiation sensor 256 within a period of time known as anintegration time. In one embodiment, thermal imager 252 may comprise anarray of radiation sensors 256 in the form of microbolometers or otherlike sensors. In other embodiments, radiation sensor may 254 maycomprise any type of known semiconductor image sensing array such asspecially doped CMOS image sensors. Other known image sensingtechnologies that can be used to determine the amount of radiationincident at a plurality of positions on a focal plane can be used.

Typically, radiation sensors 256 generate an analog output signal. Theanalog output of each is optionally amplified by an analog amplifier(not shown) and analog processed by an analog signal processor 264 toreduce any output amplifier noise of image sensor 252. The output ofanalog signal processor 264 is converted to a captured digital imagesignal by an analog-to-digital (A/D) converter 266.

The digitized image signal is optionally temporarily stored in a memory270, and is then processed using a programmable digital signal processor272. Digital signal processor 272 creates digital images of the field ofview 238. These digital images can be adapted for display on, forexample, a viewfinder display 274 or other exterior display 276.Viewfinder display 274 and exterior display 276 can comprise, forexample, a color liquid crystal display (LCD), organic light emittingdisplay (OLED) also known as an organic electroluminescent display(OELD) or other type of video display or any other known form of videoimage display. Alternatively, a communication system 280 can be used tosend the digital images to an external device 282 such as a wirelesslyconnected viewfinder, a targeting system, remote signal analysis systemsor viewing or control equipment at a remote command and control center.

Optionally, digital signal processor 272 uses the initial images tocreate archival images of the scene. Archival images are typically highresolution images suitable for storage, reproduction, and sharing.Archival images are optionally compressed using the PEG standard andstored in a data memory 278.

In operation, control system 290 sends signals to a timing generator 292indicating that images are to be captured. Timing generator 292 canprovide signals that can be used by various elements of thermal imagingsystem 248 to control image capture, digital conversion, compression,and storage operations. Thermal imager 252 is optionally driven fromtiming generator 292 by way of an image sensor driver 294. Controlsystem 290, timing generator 292 and image sensor driver 294 cooperateto cause image sensor 252 to determine an amount of radiation incidenton each of radiation sensors 256 across an integration time that iseither fixed or variable. After the integration time is complete animage signal is provided to analog signal processor 264 having analogsignals indicative of the radiation sensed at each of radiation sensors264 during the integration time. These analog signals are processed asdescribed in greater detail above.

The ability of a radiation sensor 256 to generate a signal that isrepresentative of the amount of radiation incident on the radiationsensors during an integration time is not infinite. Instead, the abilityof a radiation sensor 256 to sense radiation is limited by a lowerresponse threshold and an upper response threshold. The lower responsethreshold can be, for example, an exposure level at which the inherentsignal to noise properties of a radiation sensor 256 and the electroniccircuitry designed to extract signal information from radiation sensor256 approaches a threshold signal to noise ratio of the exposingradiation. Accordingly, when radiation sensor 256 is exposed toradiation that is below the lower response threshold, it becomesdifficult to ensure that the signal received from the radiation sensor264 accurately represents the relative intensity of radiation incidenton the sensor within the integration time.

Similarly, the upper response threshold is the light exposure levelwhere it becomes difficult to ensure that the signal received from aradiation sensor 256 accurately represents the relative intensity of theradiation incident on radiation sensor 256 within the integration time.

It will be appreciated that more image detail can be visually obtainedfrom a captured image that includes large contrast differences. Suchlarge contrast differences are lost however, when an image includes alarge proportion of image information from radiation sensors 256 thathave been exposed to light above the upper threshold or below the lowerthreshold. Accordingly, integration times are typically adjusted to helpensure that radiation sensors 264 are exposed to radiation that isgenerally between a lower threshold and an upper threshold for theradiation sensors. However, in some low radiation scenes, such as atnight there may be insufficient ambient illumination to allow imagecapture without a signal to noise ratio in the image that is too high toallow for accurate observation of a field of view. Illumination of thefield of view is therefore required.

Lasers are appropriate for illumination purposes particularly where itis desirable to project illumination at distance down range and withincontrollable wavelengths. However lasers themselves may introduce noiseinto the field of view. Of particular concern, is a condition known asspeckle. Speckle arises when coherent light reflects from more than onedifferent point in a field of view 238 in a manner that coherentlycombines at a point of observation.

FIG. 6 illustrates one hypothetical example of a speckle pattern thatmay exist in a field of view of a sensor 252. Speckle is typicallyobserved as a pattern 310 of darker spots such as darker spot 312 andlighter spot such as lighter spot 314 in a uniformly laser illuminatedfield of view 238 having at least one rough surface. Typically, thespeckle pattern 310 for a given field of view is generally static whileobservation and illumination remain constant.

It will be appreciated that such a pattern 310 of speckle can make itparticularly difficult to determine the differences between contrastpatterns in the image that are a product of the objects in field of view238 and contrast patterns in the image that are a product of speckle.

However, laser system 100, unlike the illuminators of the prior art isadapted to operate in a manner that reduces the appearance of speckle inthe field of view 238. FIG. 7 illustrates a flow chart of a method bywhich this is done.

As is shown in FIG. 7, in this embodiment, an integration time isdetermined for an imaging system 248 that is to be used with lasersystem 100 (step 320). In certain embodiments, the integration time canbe predetermined such as where it is known that laser system 100 will beused with a specific imaging system 248 or where it is known that lasersystem 100 will be used under certain conditions that require a specificintegration time. The integration time or parameters that may be relatedto the integration can also be user entered or selected by way of userinput system 110. In other embodiments, integration time can bedetermined automatically by communication between laser system 100 andimaging system 248. For example, communication system 280 of imagingsystem 248 can communicate with communication system 116 so controlsystem 290 can provide data from which an integration time used bythermal imaging system 248 can be determined.

A first current is then supplied to semiconductor laser 180 sufficientto cause semiconductor laser 180 to emit a beam of laser light 184having a first transverse mode structure (step 322).

The current applied to semiconductor laser 180 is then modulated acrossa range of current levels during the determined integration time (step326). The modulation of the current is determined so that semiconductorlaser 180 will emit light having a first transverse mode structureduring a first portion of the range of current levels and a secondtransverse mode structure during a second portion of the range ofcurrent levels. A change in transverse mode structure may take the formof a change in the number of transverse modes or the relative portion ofthe overall intensity of a laser beam 122 formed by individual ones ofmore than one simultaneously emitted transverse modes. Laser beam 122has an angular emission profile that is a function of the transversemode structure. The direction of higher intensity emissions in theangular emission profile change with the transverse mode structure. Thischanges the relative angle of incidence of an illuminating light on thefield of illumination 237 shifting the speckle pattern as will now bedescribed in greater detail with reference to FIGS. 8-10.

FIG. 8 shows a first example of a plurality of radiation sensors 256a-256 k on which a portion 316 a of speckle pattern 310 is formed asshown in FIG. 6. Portion 316 a represented in FIG. 8 by numericalpattern of positive and negative numbers. This pattern of positive andnegative numbers are representative of the intensity variation at eachradiation sensors 256 a-256 k due to portion 316 a of speckle pattern310. Positive numbers are used to denote radiation sensors 256 on whichcoherent light from field of view 238 combines to increase the amount oflight incident on a radiation sensor 256 during an integration time.Negative numbers are used to denote radiation sensors 256 on whichcoherent light from field of view 238 combines to decrease the amount oflight incident on a radiation sensor 256 during an integration time.Different integers are used to represent potential intensity differencesin the light sensed by radiation sensors 256 a-256 k during anintegration time caused by the speckle.

As is also shown in FIG. 9, when the current supplied semiconductorlaser 180 transitions from the first range of current levels to thesecond range of current levels semiconductor laser 180 generates adivergent laser light 184 having a different transverse mode structure.This causes the speckle pattern 310 to shift, in this embodiment to theright, such that portion 316 a is repositioned as shown as portion asillustrated in FIG. 9.

As is shown in FIG. 10, this shift has a number of effects net effectsover the integration time. For example, radiation sensors 256 b and 256c, 256 j and 256 k experience a modest increase in sensed radiation overthe integration time while radiation sensors 256 e and 256 h experiencea modest decrease in sensed radiation. Radiation sensors 256 f and 256 gexperience a modest net increase in sensed radiation despite receivinglight at a bright spot in the portion 316 a/b of speckle pattern 310 atleast during part of the integration time.

By way of comparison, FIG. 11 illustrates the net effects of portion 316a of speckle pattern 310 on radiation sensors 256 a-256 k using a priorart laser illumination system that does not provide the changing thetransverse mode structure. As can be seen from this, portion 316 acauses a speckle pattern over the integration time that has much higherabsolute intensities as well as having greater relative differencesbetween brighter areas and darker areas such as between radiation sensor256 f and 256 g.

It will be appreciated that by shifting the structure of transversemodes in laser beam 120 illuminating at least a part of field of view238 during the integration time of imaging system 248 the impact ofspeckle is averaged across multiple radiation sensors and the relativeimpact of speckle is greatly reduced.

In some embodiments, a ridge width or distance between transversesidewalls of an active region in a semiconductor laser 180 is selectedto provide transverse mode structures that are different when energizedin at least two different ranges of current in order to provide thedesired shift. For example, a ridge with a width of between about 1 and2 wavelengths of a light emitted as a laser beam 122 by semiconductorlaser 180 can be used for this purpose.

Additionally, the selection insulating material adjacent to transversesidewalls of an active region in a semiconductor laser 180 is used tofacilitate transitions in transverse mode structures to provide thedesired shift. For example, semi-insulating materials such as indiumphosphide can be used to achieve transverse mode structures having afirst characteristics, while di-electric materials such as silicondioxide or silicon nitride can be used to achieve transverse modestructures having second characteristics that are different from thefirst characteristics. In some embodiments the use of di-electricmaterials can more effectively lead to a change in transverse modestructure than the use of semi-insulating materials such as indiumphosphide on semiconductor lasers having equal ridge widths.

In addition, the thickness of the semi-insulating material in thepresence of a metal or a semi-conductor on the outside of thesemi-insulating material can also be used to influence thecharacteristics of the transverse mode structure.

Returning to FIG. 7 it will be appreciated that the method can includethe optional step of determining a modulation function (step 324). Themodulation function can define the amplitude and time rate of change ofthe current supplied to semiconductor laser 180. The modulation functioncan also define a shape of the waveform used in modulation. In someembodiments, the modulation frequency used to apply current intosemiconductor laser 180 may induce temperature changes withinsemiconductor laser 180 such that different transverse mode structurescan be reached based as a product of heating and cooling of thesemiconductor laser 180 within the integration time. Such heating andcooling may occur to a greater extent in response to lower frequencymodulation while occurring to a lesser extent in response to higherfrequency modulation.

As is noted above, semiconductor laser 180 can take the form of a laserthat emits light between 2 um and 30 um. However, specific wavelengthsof light within this range may be particularly useful for achievingdesired results such as illumination over a particular ranges.Conventionally selection of a semiconductor laser 180 for use inilluminating applications and in particular in illuminating applicationsover particular ranges has been made based primarily on the transmissionefficiency of the laser in expected environmental conditions.

This would suggest that different types of semiconductor lasers 180emitting different wavelengths are necessary for similar applicationsand that the selection of semiconductor laser 180 should be made basedupon anticipated use cases. This leads to unnecessary redundancy,reduced overall performance and expense.

Instead, the inventor has discovered that a system approach to selectingsemiconductor lasers 180 for use in applications such as long rangeillumination can yield superior results. In particular, the inventornotes that the fundamental upper power limit of semiconductor lasers 180having certain wavelengths is greater than that of semiconductor lasers180 having other wavelengths. For example, the fundamental upper powerlimit of semiconductor lasers 180 having a wavelength of about 4.0 um issubstantially lower than the fundamental upper power limit ofsemiconductor lasers having a wavelength of greater than 4.6 um. Thispower advantage can offset or nearly offset the efficiency advantagesprovided by lower powered semiconductor lasers 180 at all but the mostextreme environmental conditions. Additionally in some circumstancesefficiency advantages of lower powered laser system can be completelyoffset where, for example, sensing equipment such as imaging systems 248are more sensitive to wavelengths that are greater than about 4.6 umthan other wavelengths.

It will be appreciated from this that selection of semiconductor lasers180 for particular applications or groups of applications can be basedupon the response of imaging system 248 including optics such as lenssystem 250, or filters or other optical components, the atmospherictransmission characteristics, and the output power of semiconductorlaser 180.

The drawings provided herein may be to scale for specific embodimentshowever, unless stated otherwise these drawings may not be to scale forall embodiments. All block arrow representations of heat flow areexemplary of potential thermal patterns and are not limiting except asexpressly stated herein.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

1. A laser system comprising: a semiconductor laser adapted to emit abeam coherent light when supplied with an electrical current; a drivingcircuit adapted to supply a first current to the semiconductor laser andto modulate the current supplied to semiconductor across a range ofcurrent levels within a determined integration time; wherein the currentis modulated so that semiconductor laser will emit light having a firsttransverse mode structure during a first portion of the range of currentlevels and a second transverse mode structure during a second portion ofthe range of current levels causing a shift in the position of a specklepattern during the integration time that reduces the appearance ofspeckle.
 2. The system of claim 1, wherein a change in transverse modestructure takes the form of a change in the number of transverse modesin the laser beam.
 3. The system, of claim 1, wherein the change intransverse mode structure comprises a change in the relative portion ofthe overall intensity of a beam formed by individual ones of more thanone simultaneously emitted transverse modes.
 4. The system, of claim 1,wherein the laser beam has an angular emission profile that is afunction of the transverse mode structure and wherein the direction ofhigher intensity emissions in the angular emission profile change withthe transverse mode structure.
 5. The system of claim 1, wherein thelaser has a ridge width selected to provide transverse mode structuresthat are different when energized in at least two different ranges ofcurrent in order to provide the shift in transverse mode structure. 6.The system of claim 5, wherein the ridge width is between about 1 and 2wavelengths of a light emitted as a laser beam by the semiconductorlaser.
 7. A method for operating a laser system: determining anintegration time for an imaging system to be used with the laser system;supplying a current to a semiconductor laser used in the laser system;modulating the current supplied to the semiconductor laser across arange of current levels during the determined integration time whereinthe current is modulated so that the semiconductor laser will emit lighthaving a first transverse mode structure during a first portion of therange of current levels and a second transverse mode structure during asecond portion of the range of current levels causing a shift in theposition of a speckle pattern during the integration time that reducesthe appearance of speckle.
 8. The method of claim 7, wherein a change intransverse mode structure takes the form of a change in the number oftransverse modes in the laser beam.
 9. The method of claim 7, whereinthe change in transverse mode structure comprises a change in therelative portion of the overall intensity of a beam formed by individualones of more than one simultaneously emitted transverse modes.
 10. Themethod of claim 7, wherein the laser beam has an angular emissionprofile that is a function of the transverse mode structure and whereinthe direction of higher intensity emissions in the angular emissionprofile change with the transverse mode structure.
 11. The method ofclaim 7, wherein the semiconductor laser has a ridge width selected toprovide transverse mode structures that are different when energized inat least two different ranges of current in order to provide the shiftin transverse mode structure.
 12. The system of claim 11, wherein theridge width is between about 1 and 2 wavelengths of the light emitted bythe semiconductor laser.